The latest test of America's controversial missile defence system was a success, the Pentagon says.

A ground-launched interceptor missile hit a dummy warhead fired over the south Pacific on Tuesday morning, the third successful hit in five attempts.

Scientists have raised doubts about the performance of the radar system in bad weather

The test was originally scheduled for last Saturday but was postponed twice because of bad weather.

The tests have been criticised for their cost - the bill for each is $100m - and for being unrealistic.

President George W Bush and the US military argue the tests are needed so America can develop a protective shield against missiles launched by rogue states.

But Russia and China oppose the plan, saying it would breach existing arms control agreements.

A Pentagon spokeswoman said these tests were just a beginning.

"It's just one in a long line of tests we intend to conduct to test a layered missile defence system," Cheryl Irwin told the AFP news agency.

Dummy warhead

The test involved two rockets. One, launched from California, carried a dummy warhead and a decoy balloon.

A ground-based radar, spotting the objects, guided a second missile, the interceptor, towards the warhead.

The interceptor, launched from Kwajalein Atoll in the Marshall Islands, closed on the dummy at 26,000 km/h (15,000 mph).

As these are tests and safety is an important factor, the Pentagon's rules state that the sky must be clear enough to track the missile visually.

Bad weather meant this has not been the case in recent days.

In wartime, weather would not be a consideration. However, some scientists have raised doubts about the performance of the radar system in bad weather.

Rain and fog

Water in the atmosphere absorbs electromagnetic waves, including those which the so-called X-band radar uses to track missiles.

The frequencies of the X-band system are less affected by water than some others. But it is unclear what effect rain and fog would have on the ability of the radar to discriminate between a target and a decoy.

The Union of Concerned Scientists, which recently released a report on the missile defence system, says that several aspects of the tests suggest they have been designed to maximise the chances of success.